一、什么是AsyncTask

AsyncTask是輕量級的用于處理簡單的異步任務(wù)，不需要借助線程和Handler即可實(shí)現(xiàn)。除了之外還有以下方法解決線程不能更新UI組件問題；

• 使用Handler實(shí)現(xiàn)線程間通訊
• Activity.runOnUIThread(Runnable);
• View.post(Runnable)
• View.postDelayed(Runnable);

二、AsyncTask的使用方法

1.創(chuàng)建AsyncTask的子類，并為三個泛型指定類型的參數(shù)，如果不需要制定則Void.
2.根據(jù)需要調(diào)用onPreExecute()，onPostExecute(Void aVoid)，doInBackground(Void... voids)，onProgressUpdate(Void... values) 等方法
3.調(diào)用AsyncTask子類的execute(Param params)執(zhí)行耗時任務(wù)

  PAsyncTask pAsyncTask=new PAsyncTask();
  pAsyncTask.execute("廣州");


    private class PAsyncTask extends AsyncTask<String,String,String>{

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
        }

        @Override
        protected void onProgressUpdate(String... values) {
            super.onProgressUpdate(values);
        }

        @Override
        protected String doInBackground(String... strings) {
            return null;
        }
    }

四、AsyncTask機(jī)制原理

  /**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     *
     * @hide
     */
    public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);

        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    postResultIfNotInvoked(get());
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

此處是AsyncTask的構(gòu)造方法，創(chuàng)建了WorkerRunnable、FutureTask 的實(shí)例mWorker和mFuture，其中mWorker實(shí)現(xiàn)了call()的接口，處理doInBackground()數(shù)據(jù)，mFuture則是實(shí)現(xiàn)了Runnable接口的子線程，里面調(diào)用了mWorker的call()接口

 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
            }
        }

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

這里先是在UI線程中執(zhí)行了onPreExecute（）； exec.execute(mFuture);方法中exec是實(shí)現(xiàn)了Executor接口的（如下）, r.run();是實(shí)現(xiàn)了Runable接口的mFuture子線程，具體執(zhí)行子線程實(shí)在線程隊(duì)列mTasks 中執(zhí)行。

  private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

而后在postResult（）方法中使用Handler發(fā)送消息通知UI線程

 private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

總結(jié)：

• AsyncTask使用了FutureTask和Handler的結(jié)合，其中FutureTask實(shí)現(xiàn)了runnable接口，SerialExecutor則采用線程池的方式管理線程。
• AsyncTask在代碼上比Handler要輕量級別，但實(shí)際上比Handler更耗資源，因?yàn)锳syncTask底層是一個線程池，而Handler僅僅就是發(fā)送了一個消息隊(duì)列。但是，如果異步任務(wù)的數(shù)據(jù)特別龐大，AsyncTask線程池比Handler節(jié)省開銷，因?yàn)镠andler需要不停的new Thread執(zhí)行。
• AsyncTask的實(shí)例化只能在主線程，Handler可以隨意，只和Looper有關(guān)系,因此為什么開發(fā)中比較少用AsyncTask，而Handler+Runnable的方式更加靈活多變來適應(yīng)不同的業(yè)務(wù)需求。

五、AsyncTask注意事項(xiàng)

• 必須在UI線程中創(chuàng)建AsyncTask的實(shí)例
• 在UI線程中調(diào)用AsyncTask的execute();
• AsyncTask只能被執(zhí)行一次，多次調(diào)用會引發(fā)異常